The present invention concerns image printing. It has particular application to the image filtering that is employed to enhance feature rendering in half-toned images.
In most cases a digitally stored image is represented as pixel values that a device printing the image should attempt to approximate as closely as possible. But obtaining the desired fidelity is complicated by several factors. In the case of an ink-jet printer, for instance, the range of intensities available for a given color component at any given pixel site is merely binary. That is, an ink dot of the given color component can be either deposited or not: there typically is no choice in the size of the ink dot that can be deposited. Therefore, the greater value choice that the stored image's finer value quantization affords is simulated by half-toning, in which the finer-resolution value intended at a particular pixel determines the probability that an ink drop will be deposited at that pixel. Of course, half-toning affects fidelity adversely.
Additionally, the relationship between the apparent color darkness and the dot-deposition probability that will produce that darkness is not linear, so "dot-gain compensation" must be performed on the incoming image in order to increase fidelity. Similar processing must also be performed to compensate for the inaccuracy in the relationship between the primary colors that the stored values represent and the colors of which each ink actually used is capable.
For these and many other reasons, a lot of image processing must be performed in order to maximize the fidelity with which the intended image is rendered. In many cases, this processing involves filtering. Sometimes, high-pass filters are employed to re-emphasize edge features, which half-toning and various other processes tend to suppress. Conversely, low-pass filters are often employed in low-contrast regions in order to suppress processing artifacts that would otherwise be particularly apparent in such regions. Indeed, some systems employ both high- and low-pass filtering the same image: regions identified as being high in contrast are subjected to high-pass filtering for edge enhancement, while regions identified as being low in contrast are subjected to low-pass filtering for smoothing.
When printers that use half-toning employ filtering, they typically use the same filter types that other image-processing applications use. That is, a "kernel" of filter co-efficients conceptually overlays a neighborhood of pixels that are centered on the pixel whose output value is to be computed, the neighborhood-pixel values are multiplied by the co-efficients that respectively overlay them, and the output value is the sum of the resultant products. The filter effect depends both on the filter co-efficients and on the kernel size. In general, a larger low-pass-filter kernel is capable of greater smoothing than a smaller filter kernel.